PHP Fatal error: Throw "Zend_Mail_Protocol_Exception" with message "501 <>: Missing or invalid local part

Hi guys, all of a sudden I get this error in my Zend program where I am trying to send an email. I have no idea what is wrong here :(

PHP Fatal error:  Uncaught exception 'Zend_Mail_Protocol_Exception' with message '501 <>: missing or malformed local part

      

        
        
        
      

    

It worked fine until a few days ago ...